Carbon molecular sieve membranes (CMSM) and their use in the separation of various gases are known in the art, e.g., from U.S. Pat. No. 4,685,940. These membranes have been used for the separation of gas mixtures resulting from various processes. The most common process to which such methods have been applied are the separation of nitrogen and oxygen from air, but the separation of various binary gas mixtures including N.sub.2, He, O.sub.2 and SF.sub.6 have also been carried out, and carbon membranes technology is becoming a promising field for a variety of industrial applications
Unfortunately, one of the reasons which have slowed down the development of carbon membranes technology for large scale industrial purposes is that it is very difficult to manufacture hollow fiber carbon membranes modules of an industrial size. Such modules should contain thousands to hundreds of thousands of hollow fibers, which are carbonized together, and which should not be either pitted or glued together, if the module is to function properly. When attempting to manufacture such modules, however, using carbonization techniques which have been successfully applied to the pyrolysis of single fibers it has been found that the resulting module contained a large amount of broken or pitted or glued fibers, and that carbonization had not taken place uniformly along the length of the fiber bundle. This resulted in modules of poor quality and, in fact, only very small numbers of fibers could be carbonized in a bundle by such conventional techniques. Typically, up to 20-40 fibers/bundle were carbonized in a 4 mm tube. No catalyst was used in such prior art processes, and the good fibers had to be picked out from a bundle including fractured fibers.
One of the reasons why the prior art processes did not produce industrially acceptable results is that such processes were developed and implemented mainly for the carbonization of yarns, cords and fabrics, in which the fibers are already in multifilament bundles, and which do not need to carbonize evenly in an enclosed space, as in the inner part of hollow fibers. The problems associated with the carbonization of bundles are different from those of hollow fibers, mainly because the integrity of the individual fiber is not as important since the yarn can tolerate breaks in individual filaments and still be strong. Also maintaining the mechanical integrity of a hollow fiber, as opposed to a solid filament is a different task.
In general, the carbonization process comprises two main stages: 1) Pyrolysis, namely thermal decomposition, of the precursor material (preferably cellulose or some regenerated cellulose); and 2) Restructure and aromatization. The process is associated with three main technological problems:
a. The prevention of tar formation and carbon yield. The formation of tars causes the sticking together of fibers leading to embrittlement, and the tar loss is expressed in terms of lower carbon yield. The maximum theoretical yield is for cellulose carbonization 44.4% by weight (ratio of carbon residue to dry cellulose precursor).
b. Processing time. The processing time is important in order to obtain an industrially acceptable throughput of the produced fiber, via a given size of carbonization kiln.
c. Fiber strength and integrity. The mechanical properties resulting from the chosen carbonization process determine the quality (integrity) of the fiber bundle and the possible uses.
It is known in the art to manufacture carbonaceous materials by the pyrolysis of cellulosic materials. Carbonaceous materials have been manufactured for many purposes, e.g., for making textile materials (U.S. Pat. No. 3,305,315 and U.S. Pat. No. 3,527,564). The art has also recognized that carbonization can be facilitated by using carbonization catalysts, such as mineral acids and acidic salts such as phosphoric acid and diammonium hydrogen phosphates (U.S. Pat. No. 3,235,323 and U.S. Pat. No. 3,305,315), by impregnating the cellulosic material prior to pyrolysis with the catalyst. Other catalysts are described in U.S. Pat. No. 3,527,564, which are used to reduce carbonization time.
However, the preparation of hollow carbon membranes with carbonization catalysts present specific problems. In hollow carbon fibers carbonization must take place uniformly both inside and outside the fiber, and pitting must be avoided because the selectivity of the membrane depends on the uniformity of the pores produced therein during carbonization. Pitting occurs immediately if the catalyst is not uniformly distributed on the fiber, due to locally catalyzed oxidation on the surface. For instance, Shindo ACS Polymer Preprints, 9, 1333 (1968)! used HCl as a catalyst, which was applied from room temperature on. This procedure results in the formation of many defects per bundle, apparently as the result of the local formation of spots of concentrated aqueous acid which is formed through the release of hydrated water during the dehydration stage.